A non-aqueous electrolyte secondary battery, which has features of a higher operating voltage and higher energy density than that of a conventional nickel cadmium secondary battery or the like, has been widely used as a power source of an electronic device. Lithium transition metal composite oxides represented by lithium cobaltate, lithium nickelate and lithium manganate or the like are used as a positive electrode active material of the non-aqueous electrolyte secondary battery.
Among these, lithium manganate has advantages of easily obtaining a raw material inexpensively since a large amount of manganese which is a constituent element exists as resources and applying little load to environment. Thereby, the non-aqueous electrolyte secondary battery using lithium manganate has been conventionally used for the application of a mobile electronic device represented by a mobile phone, a notebook computer and a digital camera or the like.
In recent years, in the mobile electronic device, the demand characteristics of the non-aqueous electrolyte secondary battery have been further increased, due to the function advancement such as the application of various functions, and use at high temperature and low temperature, or the like. The non-aqueous electrolyte secondary battery is expected to be applied to power supplies such as batteries for electric automobiles, and the battery which can follow an abrupt start and abrupt acceleration of automobiles and enables high output high-rate discharge is desired.
Therefore, an attempt has been made to make the average particle diameter of the positive electrode active material such as lithium manganate small so as to enhance the smooth insertion/desorption capacity of Li ions. For example, the following Patent Document 1 discloses a method which uses manganese oxide having an average primary particle diameter of 0.01 to 0.2 μm, mixing manganese oxide with a lithium compound or the like and firing them, and then milling them to produce lithium manganate having an average primary particle diameter of 0.01 to 0.2 μm and an average secondary particle diameter of 0.2 to 100 μm.
However, a diffusion space sufficient for the insertion and desorption of Li ions is hard to be obtained only by making the average particle diameter of the positive electrode active material small, or by controlling the average particle diameter of aggregated particles as in the above-mentioned producing method. When the positive electrode is produced using the positive electrode active material, unfortunately, the diffusion space of the Li ions is hard to be stably secured by the mixture and pasting of a binder or the like.
Therefore, an attempt exists, which positively forms a space other than a space generated in a clearance between particles in a positive electrode active material in order to expand the diffusion space of the Li ions, thereby making the positive electrode active material porous.
For example, the following Patent Document 2 discloses a producing method of a positive electrode active material for producing a mixture containing primary particles in a lithium-containing composite oxide and pore-forming particles, then removing a pore-forming particle construction material contained in the mixture to form porous particles. In addition, Patent Document 2 discloses a method for using resins such as polystyrene as the pore-forming particles, and heating the resins to 300 to 600° C. to thermally decompose the resins to remove a part thereof.
Furthermore, the following Patent Document 3 discloses lithium manganese composite oxide granulated secondary particles obtained by granulating slurry obtained by dispersing a manganese oxide fine powder, a lithium raw material and an open pore forming agent by spray drying and thereafter firing the granulated object at 700 to 900° C.    Patent Document 1: Japanese Unexamined Patent Publication No. 2002-104827    Patent Document 2: Japanese Unexamined Patent Publication No. 2005-158401    Patent Document 3: Japanese Unexamined Patent Publication No. 2004-083388